Cardiac ischemia is a condition whereby heart tissue does not receive adequate amounts of oxygen and is usually caused by a blockage of an artery leading to heart tissue. If sufficiently severe, cardiac ischemia results in an acute myocardial infarction (AMI), also referred to as a heart attack. With AMI, a substantial portion of heart muscle ceases to function because it no longer receives oxygen, usually due to significant blockage of the coronary artery. Generally, AMI occurs when plaque (such as fat, cholesterol, and calcium) builds up and then ruptures in the coronary artery, allowing a blood clot or thrombus to form. Eventually, the blood clot completely blocks the coronary artery and so heart tissue beyond the blockage no longer receives oxygen and the tissue dies. In many cases, an AMI proves fatal because too much tissue is damaged to allow continued functioning of the heart muscle. Indeed, AMI is a leading cause of death here in the United States and worldwide. In other cases, although the AMI itself is not fatal, it strikes while the victim is engaged in potentially dangerous activities, such as driving vehicles or flying airplanes, and the severe pain and possible loss of consciousness associated with AMI results in fatal accidents. Even if the victim survives the AMI, quality of life may thereafter be severely restricted.
Often AMI is preceded by episodes of cardiac ischemia that are not sufficiently serious to cause actual permanent injury to the heart tissue. Nevertheless, these episodes are often precursors to AMI. Episodes of cardiac ischemia may also trigger certain types of arrhythmias that may prove fatal, particularly ventricular fibrillation (VF) wherein the ventricles of the heart beat chaotically, resulting in little or no net flow of blood from the heart to the brain and other organs. Indeed, serious episodes of cardiac ischemia (referred to herein as acute myocardial ischemia) typically result in either a subsequent AMI or VF, often within one to twenty-four four hours, sometimes within only a half an hour or less. Accordingly, it would be highly desirable to provide a technique for reliably detecting acute myocardial ischemia so that the victim may be warned and medical attention sought. If properly warned, surgical procedures may be implemented to locate and remove the growing arterial blockage or anti-thrombolytic medications may be administered. At the very least, advanced warning would allow the victim to cease activities that might result in a fatal accident. Moreover, in many cases, AMI or VF is triggered by strenuous physical activities and so advanced warning would allow the victim to cease such activities, possibly preventing AMI or VF from occurring.
Many patients at risk of cardiac ischemia have pacemakers, ICDs or other medical devices implanted therein. Accordingly, techniques have been developed for detecting cardiac ischemia using implanted medical devices. In particular, techniques have been developed for analyzing intracardiac electrogram (IEGM) signals in an effort to detect cardiac ischemia. See, as examples, the following U.S. Pat. Nos. 5,113,869 to Nappholz; 5,135,004 to Adams et al.; 5,199,428 to Obel et al.; 5,203,326 to Collins; 5,313,953 to Yomtov et al; 6,501,983 to Natarajan, et al.; 6,016,443, 6,233,486, 6,256,538, and 6,264,606 to Ekwall; 6,021,350 to Mathson; 6,112,116 and 6,272,379 to Fischell et al; 6,128,526, 6,115,628 and 6,381,493 to Stadler et al; and 6,108,577 to Benser. Most IEGM-based ischemia detection techniques seek to detect ischemia by identifying changes in the elevation of the ST segment of the IEGM that occur during cardiac ischemia. The ST segment represents the portion of the cardiac signal between ventricular depolarization (also referred to as an R-wave or QRS complex) and ventricular repolarization (also referred to as a T-wave). The QRS complex usually follows an atrial depolarization (also referred to as a P-wave.) Strictly speaking, P-waves, R-waves and T-waves are features of a surface electrocardiogram (EKG). For convenience and generality, herein the terms R-wave, T-wave and P-wave are used to refer to the corresponding internal signal component as well.
A significant concern with any cardiac ischemia detection technique that relies on changes in the ST segments is that systemic influences within the patient can alter the ST segment. For example, hypoglycemia (low blood sugar levels) and hyperglycemia (high blood sugar levels) can both affect ST segment elevation. In addition, electrolyte imbalance, such as hypokalemia (low potassium levels) or hyperkalemia (high potassium levels) can affect the ST segment. Certain anti-arrhythmic drugs can also affect the ST-segment.
Accordingly, alternative techniques for detecting cardiac ischemia have been developed, which do not rely on ST segment elevation. One such technique is set forth in U.S. patent application Ser. No. 10/603,429, entitled “System And Method For Detecting Cardiac Ischemia Using An Implantable Medical Device”, of Wang et al., filed Jun. 24, 2003, which is incorporated by reference herein. Rather than examine the ST segment, the technique of Wang et al. instead examines post-T-wave segments, i.e. that portion of the cardiac signal immediately following the T-wave. In one example, the onset of cardiac ischemia is identified by detecting a sharp falling edge within post-T-wave signals. A warning is then provided to the patient. The warning preferably includes both a perceptible electrical notification signal applied directly to subcutaneous tissue and a separate warning signal delivered via short-range telemetry to a handheld warning device external to the patient. After the patient feels the internal warning signal, he or she holds the handheld device near the chest to receive the short-range telemetry signal, which provides a textual warning. The handheld warning device thereby provides confirmation of the warning to the patient, who may be otherwise uncertain as to the reason for the internally generated warning signal. Another technique for detecting cardiac ischemia based on T-waves is set forth in U.S. patent application Ser. No. 10/603,398, entitled “System And Method For Detecting Cardiac Ischemia Based On T-Waves Using An Implantable Medical Device”, of Min et al., filed Jun. 24, 2003, which is also incorporated by reference herein. With the technique of Min et al., cardiac ischemia is detected based either on the total energy of the T-wave or on the maximum slope of the T-wave. Again, if ischemia is detected, a warning signal is provided to the patient.
Hence, various cardiac ischemia detection techniques have been developed that exploit T-waves. Although these techniques are effective, it is desirable to provide still other T-wave-based ischemia detection techniques. It is also desirable to provide techniques that exploit deviations in the ST segment as well as changes in T-waves to provide further improvements cardiac ischemia detection. In particular, it is highly desirable to identify particular changes in T-waves that can be used to distinguish deviations in the ST segment caused by cardiac ischemia from changes caused by hypoglycemia or hyperglycemia or other systemic affects so as to improve the reliability and specificity of ST segment-based ischemia detection.
Although the detection of cardiac ischemia is of paramount importance since cardiac ischemia may be a precursor to a potentially fatal AMI or VF, it is also highly desirable to detect hypoglycemia or hyperglycemia, particularly within diabetic patients. Indeed, hypoglycemia is believed to be the cause of death in about three percent of insulin-treated diabetic patients. The putative mechanism for death due to hypoglycemia is a hypoglycemia-induced prolongation of the QT interval of the intracardiac electrogram (IEGM), which increases the risk of malignant ventricular tachycardia. See, for example, Eckert et al., “Hypoglycemia Leads to an Increased QT Interval in Normal Men”, Clinical Physiology, 1998, Volume 18, Issue 6, Page 570 and also Heller, “Abnormalities of the Electrocardiogram during Hypoglycaemia: The Cause of the Dead in Bed Syndrome”, Int. J. Clin. Pract. Suppl. 2002 July; (129): 27-32. Note that QT interval represents the portion of the IEGM between the beginning of ventricular depolarization and the peak of ventricular repolarization.
Hypoglycemia is also a serious and frequent problem in patients suffering hyperinsulinism, wherein the body generates too much insulin, thereby triggering episodes of hypoglycemia even if an otherwise sufficient amount of sugar or other glucose-generating substances are ingested. Medications appropriate for addressing hyperinsulinism included sulfonylureas, meglitinides, biguanides, thiazolidinediones, or alpha glucosidase inhibitors.
In adults, if not treated properly, severe hypoglycemia may result in coma and irreversible brain damage. McCarthy et al., “Mild hypoglycemia and impairment of brain stem and cortical evoked potentials in healthy subjects.” Department of Pediatrics, Yale University School of Medicine, New Haven, Conn. 06510.
Even in cases where hypoglycemia does not cause severe consequences, it is often the limiting factor in achieving good glycemic control in patients with diabetes, particular insulin-depended diabetics. In this regard, patients sometimes refrain from taking prescribed dosages of insulin for fear that the insulin might trigger an episode of hypoglycemia, which can be unpleasant. Failure to take the prescribed insulin prevents the patient from maintaining glycemic levels within a healthy range, thus often leading to additional health problems.
Hyperglycemia, in contrast, is a condition characterized by abnormally high blood glucose levels. Often, hyperglycemia arises due to a lack of insulin within insulin-dependent diabetics. Hyperglycemia within diabetics can lead to ketoacidosis (i.e. diabetic coma), which can be fatal. Briefly, ketoacidosis occurs if the body lacks sufficient insulin to properly process the high blood glucose levels associated with hyperglycemia. Without sufficient insulin, the body cannot process glucose for fuel and hence breaks down fats to use for energy, yielding ketones as waste products. However, the body cannot tolerate large amounts of ketones and tries to eliminate the ketones through urine. Often, though, the body cannot eliminate the ketones and hence ketones build up in the blood leading to ketoacidosis. Excessively high ketone levels in the blood can be fatal.
Diabetic patients, hence, need to frequently monitor blood glucose levels to ensure that the levels remain within acceptable bounds and, for insulin dependent diabetics, to determine the amount of insulin that must be administered. Conventional techniques for monitoring blood glucose levels, however, leave much to be desired. One conventional technique, for example, requires that the patient draw blood, typically by pricking the finger. The drawn blood is then analyzed by a portable device to determine the blood glucose level. The technique can be painful and therefore can significantly discourage the patient from periodically checking blood glucose levels. Moreover, since an external device is required to analyze the blood, there is the risk that the patient will neglect to keep the device handy, preventing periodic blood glucose level monitoring. For insulin-dependent diabetics, failure to properly monitor blood glucose levels can result in improper dosages of insulin causing, in extreme cases, severe adverse health consequences such as a ketoacidotic diabetic coma, which can be fatal. Accordingly, there is a significant need to provide reliable hypo/hyperglycemia detection techniques, which do not rely on the patient to monitoring his or her own glucose levels and which does not require an external analysis device.
In view of the many disadvantages of conventional external blood glucose monitoring techniques, implantable blood glucose monitors have been developed, which included sensors for mounting directly within the blood stream. However, such monitors have not achieved much success as the glucose sensors tend to clog over very quickly. Thus, an implantable device that could continually and reliably measure blood glucose levels without requiring glucose sensors would be very desirable. Moreover, as with any implantable device, there are attended risks associated with implanting the blood glucose monitor, such as adverse reactions to anesthetics employed during the implantation procedure or the onset of subsequent infections. Hence, it is desirable to provide for automatic hypo/hyperglycemia detection using medical devices that would otherwise need to be implanted anyway, to thereby minimize the risks associated with the implantation of additional devices. In particular, for patients already requiring implantation of a cardiac stimulation device, such as a pacemaker or ICD, it is be desirable to exploit features of electrical cardiac signals.
It is now known that hypoglycemia can be detected based on observation of changes in the QT interval observed within an ECG (based on studies involving experimental hypoglycemia within adults with type 1 diabetes, i.e. insulin-dependent diabetes), as well as based on observation of dispersion of QT intervals within the ECG (based on studies involving experimental hypoglycemia within adults with type 2 diabetes, i.e. non-insulin dependent diabetes.) See, e.g., Landstedt-Hallin et al., “Increased QT dispersion during hypoglycaemia during hypoglycaemia in patients with type 2 DM.” Studies in diabetics have also shown that hypoglycemia can be detected based on observation of a significant lengthening of the QTc interval occurring during spontaneous nocturnal hypoglycemia. See, Robinson et al., “Changes In Cardiac Repolarization During Clinical Episodes Of Nocturnal Hypoglycaemia In Adults With Type 1 Diabetes” Diabetologia. February 2004; 47(2):312-5. Epub 08 Jan. 2004. The QTc interval is an adjusted version of the QT interval that has been corrected to a heart rate of 60 beats per minute (bpm). See, also, U.S. Pat. No. 6,572,542 to Houben, et al., entitled “System and Method for Monitoring and Controlling the Glycemic State of A Patient”, which describes a technique exploiting a combination of ECG signals and electroencephalogram (EEG) for the detection of hypoglycemia.
See also U.S. Pat. No. 5,741,211 to Renirie, entitled “System And Method For Continuous Monitoring Of Diabetes-Related Blood Constituents.” According to Renirie, in a non-diabetic subject, a glucose load, as results from food intake, leads to an increase in plasma glucose. In turn, the pancreas produces an increase in blood insulin. Following an increase in insulin, there is a cellular membrane change which results in infusion of potassium into the cells, and a subsequent decrease in blood potassium along with glucose uptake. The lowered extracellular potassium, or blood potassium, shortens the cardiac monophasic action potential, and produces a steeper monophasic action potential upstroke. This in turn results in observable ECG changes, such as the development of U-waves, ST segment depression, and in particular a shortening of the T-wave amplitude and a small increase in the R wave. Renirie is primarily directed to a Holter-type external monitor that analyzes the ECG but has some speculative discussions pertaining to implantable devices as well.
Although hyper/hypoglycemia detection techniques based on analysis of the ECG are somewhat helpful, there is a significant need to develop IEGM-based techniques for detecting and distinguishing between hyperglycemia and hyperglycemia, as well as improved IEGM-based techniques for detecting cardiac ischemia.
These and other problems were solved by the invention of the parent application cited above. Briefly, using the techniques of the parent application (which are also described herein-below) hypoglycemia is detected based on a change in ST segment elevation along with a lengthening of either the interval between the QRS complex and the end of a T-wave (QTmax) or the interval between the QRS complex and the end of the T-wave (QTend). Hyperglycemia is detected based on a change in ST segment elevation along with minimal change in QTmax and in QTend. Ischemia is detected based on a shortening QTmax, alone or in combination with a change in ST segment elevation. Alternatively, cardiac ischemia is detected based on a change in ST segment elevation combined with minimal change in QTend. By exploiting QTmax and QTend in combination with ST segment elevation, changes in ST segment elevation caused by hypo/hyperglycemia can be properly distinguished from one another and from changes caused by ischemia.
The following table summarizes changes in the ST segment, QTmax and QTend in response to hypoglycemia, hyperglycemia and cardiac ischemia that are exploited by the technique of the parent application.
TABLE IST SegmentQTmaxQTendHypoglycemiaSignificantLengthensLengthensdeviationHyperglycemiaSignificantLittle or noLittle or nodeviationchangechangeIschemiaSignificantShortensLittle or nodeviationchangeNormalNo significantNo significantNo significantdeviationdeviationdeviation
Another useful technique is set forth in U.S. patent application Ser. Number 2004/0077962 of Kroll, published Apr. 22, 2004, entitled “System and Method for Monitoring Blood Glucose Levels Using an Implantable Medical Device.” The technique of Kroll is directed to detecting blood glucose levels based on IEGM signals sensed by an implantable medical device. Briefly, blood glucose levels are determined by an implantable device based on IEGM signals by detecting and examine a combination of T-wave amplitude fraction and QTc interval. The technique may also be used to detect hypoglycemia based on changes in blood glucose levels.
Yet another useful technique is set forth in U.S. patent application Ser. No. 11/117,624 of Bharmi, filed Apr. 27, 2005, entitled “System and Method for Detecting Hypoglycemia Based on a Paced Depolarization Integral Using an Implantable Medical Device,” , which is assigned to the assignee of the present invention and is incorporated by reference herein. Briefly, techniques are provided therein specifically for detecting and tracking hypoglycemia. In one example, an implantable medical system tracks changes in a paced depolarization integral (PDI). A significant increase in PDI over a relatively short period of time indicates the onset of hypoglycemia. Upon detection of hypoglycemia, appropriate warning signals are generated to alert the patient. Certain therapies automatically provided by the implantable system may also be controlled in response to hypoglycemia. For example, if the patient is an insulin-dependent diabetic and the implantable system is equipped with an insulin pump capable of delivering insulin directly into the bloodstream, insulin delivery is automatically suspended until blood glucose levels return to acceptable levels. If the system includes an ICD, the ICD may be controlled to begin charging defibrillation capacitors upon detection of hypoglycemia so as to permit prompt delivery of a defibrillation shock, which may be needed if hypoglycemia triggers ventricular fibrillation.
Although the techniques described by Kroll and Bharmi as well as the techniques of the parent application are effective for detecting and distinguishing hypoglycemia and hyperglycemia, it would nevertheless be desirable to provide further improvements so as to provide improved detection specificity. By providing improved specificity in detecting hypoglycemia and hyperglycemia, any warning signals and any therapy delivered in response to hyper/hypoglycemia can be more reliably delivered. Furthermore, cardiac ischemia detection techniques of the type originally set forth in the parent application, which distinguish cardiac ischemia from hyper/hypoglycemia based on features of the IEGM, can also be more reliably performed. It is to this end that the invention of the present patent application is primarily directed. Moreover, still other aspects of the invention are directed to providing techniques for tracking changes in glycemic state so as to allow patients to achieve improved glycemic control. In particular, it is desirable to provide techniques for trending and tracking hyper/hypoglycemia in an effort to predict the onset of an episode of hypoglycemia in advance so as to warn the patient and still other aspects of the invention are directed to that end.